More on the mechanical side:
Damaged mechanical bearing surfaces can also attribute similar effects. Not
just perpendicular to the diameter of the shaft but also the 'play'(in and
out) of the rotor. Caused by: normal wear, cheap gears pulling/pushing the
shaft or out of round, worm spur/spline spur without(or worn) bushing
plates, excessive loading, over revving(exceeding intended rpm). One can
usually pick this out under a no-load comparison of motor rpm to that of a
known good motor to the suspected under the same supply. Quicky fix that
doesn't always work: a few drops of oil on bearing surfaces.

Brushes may also be of a composite that forms an oxide on the commutator if
the intended volt/amp rating is exceeded. Usually noted by a definate black
coloring and excessive arcing when in use. Unfortunately this is similar to
an out-of-round commutator.
Apply a bit of ultra fine sandpaper or gentle scrape commutator for oxide
contamination. Doubt it's worth trying to sure the commutator on a little
motor.

Have fun damnit.
"Michael A. Covington" wrote
in message ...
Thanks to all who are responding! (And - Spehro, Sam, etc. - it's good to
hear from you again. I've been away from the electronics newsgroups for a
while.) I'll point my students to this thread.

"Michael A. Covington"
wrote
in message ...
My students are having the following experience with small
permanent-magnet
DC motors:

After some kind of misfortune (overload? nobody knows), the motor starts
drawing much more current than it should. For example, a damaged motor
will
draw 1 amp at 3 volts, while the undamaged one will draw only 200 mA at
that
voltage. (Normal voltage is much higher, about 10 V.) Both motors turn
easily; the damaged one seems to run fine except for requiring excessive
current; and both have a resistance of about 1.5 ohms measured with an
ohmmeter.

This has happened to several motors.

Are the permanent magnets getting demagnetized? How? Or what else
could
be
going on? My electronics background is analog and digital but not much
about motors!


Many thanks,

Michael A. Covington - Artificial Intelligence Ctr - University of
Georgia

"In the core C# language it is simply not possible to have an
uninitialized
variable, a 'dangling' pointer, or an expression that indexes an array
beyond its bounds. Whole categories of bugs that routinely plague C and
C++
programs are thus eliminated." - A. Hejlsberg, The C# Programming
Language